1. Field of the Invention
The present invention relates generally to the field of fiber optical communications, and in particular to optical fiber amplifiers and amplification techniques.
2. Background Art
Few-mode rare-earth-doped fiber amplifiers provide gain to the fundamental (LP01) mode and a relatively small number of higher-order LPm,n modes, and are critical components in space-division-multiplexed optical transmission systems based on few-mode transmission fibers. Such systems have the potential of greatly enhancing transmission capacity and thus have recently attracted a great deal of interest.
A prior-art fiber amplifier for use in single-mode transmission typically has a step refractive-index profile, comprising a core for guiding a fundamental mode (LP01) optical signal. A prior-art amplifier fiber further includes a rare-earth-doped region having a radius that is the same or slightly smaller than the core radius. A suitable pump source is used to provide pump radiation to the rare-earth-doped region.
Transmission of light signals at higher-order modes requires a core that is larger than that of a single-mode fiber amplifier. However, an increase in core size results in significant gain differences among the supported transmission modes. Gain equalization is thus a significant issue to be addressed in the development of few-mode optical fiber amplifiers. In addressing this issue, a few-mode optical fiber amplifier design should also take into consideration the overall complexity and cost of the design.